The present invention concerns a servo-assisted steering system for a motor vehicle, said steering system comprising a steering wheel for presetting a desired steering angle for the wheels of the vehicle, driving-dynamics means for superimposing a correction angle on the steering angle, said correction angle being determined with a view toward increasing the driving stability and/or driving comfort of the motor vehicle, and a servo drive. The invention further concerns a servo drive of a servo-assisted steering system for a motor vehicle, implemented as variable torque assistance and comprising a converter that can be acted upon by a converter flow and that serves as a control device for the steering angle of the wheels of the motor vehicle, and means for determining the converter flow as a function at least of the road speed of the motor vehicle. Finally, the invention concerns a method for determining the converter flow of a servo drive implemented as variable torque assistance and forming part of a servo-assisted steering system of a motor vehicle, said servo drive comprising a converter that can be acted upon by the converter flow and that serves as a control device for the steering angle of the wheels of the motor vehicle, and the converter flow being determined as a function at least of the road speed of the motor vehicle.
In the steering systems known from the prior art, it is known to provide a servo drive for torque assistance (servo steering). The known servo drives are divided into hydraulic, electrohydraulic and electrical systems. In the case of hydraulic systems, a distinction is drawn between systems with permanently preset torque assistance (normal servo steering) and variable torque assistance, in which the degree of torque assistance is controlled as a function of the road speed of the vehicle. At low speeds, for example during parking, the amount of energy expended by a driver in operating the steering wheel is slight. With increasing road speed, a greater amount of energy is expended by the driver. Variable torque assistance gives the steering system especially easy action at low road speeds without causing it to become “soft” at higher speeds. A description of the design and operation of a servo steering system can be found in the paper “Servo drives for front- and rear-wheel steering systems in passenger vehicles,” H. Bischof, G. Dräger, W. Schleuter, a contribution to the All-Wheel Drive Conference, Haus der Technik, Essen, Nov. 28–29, 1989, pp. 1 to 16, which is incorporated herein by reference.
It is further known from the prior art to provide driving-dynamics means in steering systems in order to superimpose a correction angle on the steering angle of the wheels which is preset by the steering wheel. Driving-dynamics means are also known as driving-dynamics steering systems (DDSS). A “superimposing box” is used to superimpose the angles. The design and manner of operation of a driving-dynamics steering system is described in detail in U.S. Pat. No. 5,205,371, the disclosure of which is incorporated herein by reference. A driving-dynamics steering system improves driving dynamics, vehicle safety, and vehicle comfort. However, a power-assist function (servo steering) cannot be obtained with such a system. The correction angle that is to be added to the preset steering angle changes the actual steering angle of the wheels. The turning angle of the steering wheel preset by the driver is not altered, and remains in its preset position.
In the case of servo-assisted steering systems comprising both a servo drive for torque assistance and driving-dynamics means, the driving-dynamics means are usually arranged between the steering wheel and the servo drive. With variable torque assistance, however, this has the result that the servo drive affects the dynamics of the driving-dynamics means, especially the dynamics of an electric motor as shown in U.S. Pat. No. 5,205,371 of the driving dynamics means. The efficiency of the driving-dynamics means is not the same over the vehicle's entire range of road speeds. At higher vehicle speeds, the torque assistance is so slight that the electric motor of the driving-dynamics means has to work against very high countertorque. As a result, the electric motor cannot be accelerated as quickly and the steering interventions of the vehicle's driving-dynamics means are unable to stabilize quickly and reliably enough to increase the driving comfort of the vehicle.
It is therefore the object of the present invention, in servo-assisted steering systems comprising both driving-dynamics means and variable torque assistance, to achieve efficiency for the driving-dynamics means over the entire road-speed range of the motor vehicle.